One of widely-used methods for ion beam processing is an ion beam etching (hereinafter also called IBE) process, which etches a substrate mounted on a substrate holder by irradiating positive ions (or ion beam) to the substrate. An IBE apparatus includes an ion source (or an ion beam generation source) configured to extract positive ions from plasma by an extraction electrode, and a substrate holder disposed opposed to the ion source, and configured to hold a substrate in a vacuum. Generally, the IBE apparatus further includes an electron source (or a neutralizer) configured to emit electrons to the positive ions extracted from the ion source, a shutter device disposed between the ion source and the substrate holder and configured to block off the ion beam, and an evacuation device configured to evacuate a processing chamber to produce a vacuum in the processing chamber (Refer to U.S. Patent Application Publication No. 2002/0175296.)
A substrate is etched by the extracted ion beam. During this etching, a positive potential is induced in the substrate by the ion beam of a positive charge. The positive potential may cause damage to an insulating film formed on the substrate. Generally, the damage to a substrate element is called electrostatic damage (ESD). In particular, an element having an insulating film of a few nanometers, such as a TMR (Tunnel Magneto-Resistance) element, may undergo the electrostatic damage, and thus, it is desirable that substrate potential variations be suppressed in a processing step for such an element.
During the etching process, fluctuations of the substrate potential are suppressed by emitting electrons for neutralizing the positive potential between an extraction electrode and the substrate. The electrons for neutralization are emitted from the neutralizer located near the extraction electrode of the ion source. The amount of electrons emitted is adjusted so that the substrate potential may reach a predetermined value. The start and end of the etching process are controlled by opening and closing a shutter of the above-described shutter device. During operation of the shutter, the amounts of ion beam and electrons incident on the substrate both individually vary according to a shutter aperture. At this time, the substrate potential varies if the amounts of ion beam and electrons incident on the substrate get out of balance.
In this regard, there has been known a technique for suppressing the fluctuations of the substrate potential depending on an operation state of the shutter for blocking off the ion beam emitted from the ion source (Refer to U.S. Patent Application Publication No. 2002/0175296 and Japanese Patent Application Laid-Open No. 2012-129224).
U.S. Patent Application Publication No. 2002/0175296 discloses a technique for reducing fluctuations of the substrate potential by controlling an output from the ion source during an opening operation of the shutter. Specifically, U.S. Patent Application Publication No. 2002/0175296 discloses that, in order to solve insufficient neutralization of the substrate potential during the opening of the shutter, the amount of ion beam emission or an acceleration voltage of the ion beam is controlled to be reduced in opening and closing of the shutter.
Japanese Patent Application Laid-Open No. 2012-129224 discloses that the fluctuations of the substrate potential caused during the opening and closing operation of the shutter are suppressed by adjusting the amount of electrons (or a neutralization current) emitted from the neutralizer according to the opening and closing operation of the shutter.
However, the fluctuations of the substrate potential also occur at any time other than during the opening and closing operation of the shutter. When the ion source forms plasma with the shutter closed, the plasm is diffused through an opening of the extraction electrode into a processing space. At this time, because of a difference in mobility between electrons and ions, the electrons reach the substrate earlier, and the substrate is charged with a negative voltage during until positive ions reach the substrate. Thus, even if the shutter is closed, the substrate potential fluctuates by the influence of electrons of the plasm leaking out from the ion source.
The fluctuations of the substrate potential are caused by the fact that the shutter does not completely shield the substrate from the ion source. For example, an apparatus applied in U.S. Patent Application Publication No. 2002/0175296 is configured so that the shutter device can completely block off the ion beam and electrons from a substrate-side space in principle, and thus, when the shutter is closed, the ion beam and the electrons (i.e. the plasma) are confined in a narrow hermetically-closed space between the ion source and the shutter. In this case, since the ion source is completely isolated from the substrate as mentioned above, a minus potential is unlikely to occur on the substrate when the shutter is closed. However, actually, a gap between a movable shutter and a chamber cannot be eliminated, and thus, electrons from the ion source side may be emitted through the gap to the substrate side, and it is difficult to eliminate the occurrence of the minus potential. Also, in such an apparatus in which the movable shutter cannot isolate the ion source and the substrate, the plasm leaking out from the ion source is emitted to the substrate side through a gap between the shutter and an inner wall of the chamber, and thus, the shutter increases a distance traveled by the electrons and ions from the ion source to the substrate. Thus, a further difference arises between the time taken for the ions of the plasm leaking out from the ion source to reach the substrate and the time taken for the electrons of the plasm to reach the substrate. Therefore, the problem of instability of the substrate potential may become more noticeable.
Also, in a shutter device which does not shield a substrate-side space from a plasma formation space even when the shutter is closed as disclosed in Japanese Patent Application Laid-Open No. 2012-129224, or equivalently, in a shutter device which does not block off the routing of the plasma to the substrate even when the shutter is closed, there exists the problem of being incapable of suppressing the occurrence of the minus substrate potential.
Such charge-up of the substrate may cause dielectric breakdown of an element or the like formed on the substrate. Heretofore, this minus potential has not become a serious problem, but suppression of the minus potential becomes necessary as the element becomes finer.